When cargo is transported over large distances, the transporting time is often long, and the ambient conditions, such as outdoor temperature and weather conditions may change significantly. Accordingly, when transporting temperature sensitive cargo, it is necessary to be able to control the temperature in the cargo container in such a manner that the temperature of the cargo does not become lower or higher than a specified acceptable temperature range. To this end some cargo containers are equipped with a refrigeration system for controlling the temperature inside the cargo container. It has previously been attempted to reduce the energy consumption of such refrigeration systems. The consequence has been that the fluctuations of the temperature inside the refrigerated volume have increased, i.e. the temperature control has become less accurate than is the case in the high energy consumption systems. Accordingly, it is necessary to choose between low energy consumption or good and accurate temperature control.
US 2010/0101770 A1 discloses a method for operating a refrigeration system for a container for refrigerating chilled cargo. The method includes determining the temperature of supply air discharged into the container and the return air from the container, determining one of a requirement for heating and a requirement for cooling based on the temperatures of the return air and the supply air, activating the evaporator fan when a requirement for heating is determined and increasing the speed of the evaporator fan when increased heating is determined, and activating the compressor and the evaporator fan when a requirement for cooling is determined and increasing the power supplied to the compressor and maintaining the evaporator fan at a first speed when increased cooling is determined. Thus, in the method disclosed in US 2010/0101770 A1 the speed of the evaporator fan is controlled on the basis of the measured temperatures.
US 2010/0106303 A1 discloses a method for operating a refrigeration system for a container to pull down the temperature of cargo from an ambient temperature to a predetermined set-point temperature. Initially, an evaporator fan is operated at a first speed to supply refrigerated supply air from an evaporator to the cargo within the container. The method comprises sensing the temperature of the supply air, comparing the temperature of the supply air with a predetermined set-point temperature, and increasing the speed of the evaporator fan to a second speed faster than the first speed when the temperature of the supply air is lower than the predetermined set-point temperature to maintain the temperature of the supply air at the predetermined set-point temperature. Thus, in the method disclosed in US 2010/0106303 A1 the speed of the evaporator fan is controlled on the basis of the measured supply air temperature.